The Contribution of the Intergalactic Medium to Cosmic Microwave Background Anisotropies

We compute the power spectrum of the Cosmic Microwave Background temperatureanisotropies generated by the Intergalactic Medium. To estimate the electronpressure along the line of sight and its contribution to the Sunyaev-Zeldovichcomponent of the CMB anisotropies, we assume the non-linear baryonic densitycontrast is well described by a log-normal distribution. For model parametersin agreement with observations and for an experiment operating in theRayleigh-Jeans regime, the largest IGM contribution is expected at scalescorresponding to multipole numbers of about 2000. The amplitude is ratheruncertain and could be as large as 100-200 [mikroK]^2, comparable to thecontribution of galaxy clusters. The actual value is strongly dependent on thegas polytropic index and the amplitude of the matter power spectrum sigma_8. Atall redshifts, the largest contribution comes from scales very close to thecomoving baryon Jeans length. This scale is not yet resolved in numericalsimulations that follow the evolution of gas on cosmological scales. Theanisotropy generated by the Intergalactic Medium could make compatible theexcess of power measured by Cosmic Background Imager (CBI) on scales of l >2000 with sigma_8=0.9. Taking the CBI result as an upper limit, the polytropicindex can be constrained to < 1.5 at 2-sigma level at redshifts z ~ 0.1-0.4.With its large frequency coverage, the PLANCK satellite will be able to measurethe secondary anisotropies coming from hot gas. Cluster and IntergalacticMedium contributions could be separated by cross correlating galaxy/clustercatalogs with CMB maps. This measurement will determine the state of the gas atlow and intermediate redshifts.Comment: 22 pages, 7 figures, accepted for publication in the Astrophysical Journa